Ion beam sources have been used for a multitude of applications from space propulsion to etching and sputter deposition of films used in semiconductors and optical films. All of these applications ionize gas molecules by removing electrons to cause the gas molecule to become a positively charged ion.
The simplest of these ionizing methods was to use a filament, or thermionic emitter, to generate electrons within the ionization chamber. The electrons created by the filament collided with the gas molecules, knocking off electrons from the gas molecules to cause the molecules to become positively charged. This method, although operable, had several disadvantages. The filaments tended to have a short life. Because the filaments were thermionic emitters and were at a negative electrical potential relative to the ionized gas, material was sputtered off of the filament which caused contamination to be introduced into the ion beam.
An improvement upon the filament type of ion generation was the introduction of a hollow cathode. This eliminated the need of a filament and greatly increased the operational life. Potentials for contamination of the ion beam due to materials present in the hollow cathode were still present.
Further advances of ion beam generating devices included using a high-frequency generator coupled to either plates or coils within the chamber to ionize the gas molecules through excitation by the high-frequency energy. These materials, especially coils within the plasma field, also created contamination in the ion beam. An advancement, placing a coil outside the gas chamber helped to eliminate this contamination. However, external magnetic fields were usually required to contain the plasma within the chamber enhancing ionization efficiency and preventing arcing from the plasma to various components within the chamber. The arcing could cause a rapid degradation of the plasma and ultimate destruction of the components within the chamber. Most of the attempts to use high-frequency plasma generation also required that the generator coil be cooled by internal water means. This introduced the problem of having each end of the coil at the same potential, preferably ground potential, to prevent the high-frequency energy from being bled off to ground. Elaborate matching networks, or tight control of the length of the waveguide, or coil, were required in order to accomplish these goals.
A need, therefore, exists for being able to generate a plasma of positively charged gas molecules without contaminating the beam by the plasma touching contaminating fixturing within the chamber and without the need of water cooled coils or external magnetic fields.